Photosensitive structural body flexographic printing plate

ABSTRACT

A photosensitive constituent for a flexographic printing plate comprising a polyester film as support (B) and a photosensitive elastomer composition (C) and having an adhesive layer (A) therebetween, wherein the photosensitive elastomer composition (C) comprises, as essential components, (a) 50% by weight or more of at least one thermoplastic elastomer comprising the reaction product of a mono-vinyl-substituted aromatic hydrocarbon monomer and a conjugated diene monomer, (b) 5 to 40% by weight of at least one diene-based liquid rubber, (c) 1 to 20% by weight of at least one ethylenically unsaturated compound having 2 unsaturated bonds within its molecule, and (d) 0.1 to 3% by weight of at least one photopolymerization initiator, and wherein (1) the thermoplastic elastomer has an average ratio of vinyl bond units in conjugated diene segments thereof of 10 to 20 mol % and (2) the liquid rubber has a number average molecular weight (Mn) of 500 to 5,000 and an average ratio of vinyl bond units of 50 to 70 mol %. This is a photosensitive constituent for a flexographic printing plate which provides good fine dot-forming properties and a good printing quality in a printing process, which enables tackiness of the plate surface to be removed by post-exposure treatment with ease, while avoiding formation of cracks in the plate surface and, at the same time, has sufficiently strong adhesion between the photosensitive resin layer and its polyester film support to avoid trouble under actual printing conditions, and which is stable over time.

TECHNICAL FIELD

[0001] This invention relates to a photographic constituent for aflexographic printing plate.

BACKGROUND ART

[0002] Photosensitive constituents for a flexograpic printing plategenerally comprise a support such as a polyester film having providedthereon a photosensitive elastomer composition containing athermoplastic elastomer, at least one ethylenically unsaturated compoundand at least one radiation-responsive initiator. In addition, a thinfilm, often called a slip layer or a protective layer, is provided, ifnecessary, on the photosensitive layer for the purpose of smoothingcontact with a negative film. In making a flexographic printing platefrom such a photosensitive constituent for a flexographic printingplate, it is customary to conduct, first, UV radiation exposure all overthe constituent through the support (back exposure) to thereby form athin uniform cured layer, then to conduct imagewise exposure (reliefexposure) of the photosensitive resin layer through a negative film andwash away unexposed areas with a developing solvent to thereby obtain adesired image, i.e., a relief image which is used as a printing plate.

[0003] In a printing process by which fine photographs or the like areprinted, it is required to form extremely fine halftone dots or finelines as a relief image, but, in such cases, it is not easy to preparean adapted printing plate. For example, when relief exposure isconducted in a large exposure amount in order to form a fine reliefimage, it makes a reverse image (depressions) having a small depthwhich, upon printing, sometimes suffers the disadvantage of thespreading of ink and fails to enable reproduction of a distinct print.

[0004] In addition, there often occurs the failure that reproduction ofhalftone dots or fine lines becomes larger than the dimension of thosein the negative film, which exerts detrimental influences on printingreproducibility.

[0005] On the other hand, when the amount of relief exposure isdecreased so as to avoid such failure, photocuring of the relief imagebecomes insufficient, and hence chips are easily formed in some cases bypressure of a washing brush or the like in a developing step, and theintended image-forming properties are not obtained.

[0006] In addition, while most of dried printing plates afterdevelopment have residual tackiness on the surface thereof, suchtackiness remaining on the printing plate causes adhesion between theprinting plates to each other and, when delaminating the plates fromeach other, the relief image in some cases is damaged or extraneousmatter cannot be easily removed, which exerts detrimental influences onprinting quality. In such cases, tackiness on the surface of theprinting plates can be removed by employing the so-called post-exposuretreatment of irradiating the dried printing plate with UV radiation ofshorter wavelength as described in JP-B-2-46935 (The term “JP-B” as usedherein means an “examined Japanese patent publication”).

[0007] In this process, however, there is involved a complicatedprocedure of discreetly determining the amount of the UV radiation ofshorter wavelength. Because, if the irradiation amount of the UVradiation is insufficient, the degree of surface tack is insufficientlydecreased, whereas, if too much, there results such deterioratedphysical properties of the surface of printing plate that a number ofcracks are formed during the carrying of the plate or in the course ofprinting, with the print quality being diminished.

[0008] The obtained printing plate is fixed around a plate cylinderusing a pressure sensitive double coated tape or by drawing a vacuum soas to be subjected to a printing step. However, there must not bedelamination between the support and the cured resin due to shear stressbetween the plate cylinder and the impression cylinder during printing.In addition, while the printing plate is removed after completion ofprinting for storage and preparation for re-printing, it is necessaryfor the printing plate not to be broken and to firmly adhere to thesupport even when a large force is applied to the cured resin layer uponbeing removed by holding part of the cured resin layer.

[0009] For this purpose, good adhesion is necessary between the supportand the photosensitive resin layer and, as a general guide, an adhesionforce of 1 kg/cm or more is desirable when forcibly delaminated at anangle of 180 degrees. From this point of view, it has become customaryto provide an adhesive layer between the support and the photosensitiveresin layer to thereby strengthen adhesion between the support and thephotocured resin layer.

[0010] In general, a polyester film constituting the support and thephotosensitive layer mainly containing a rubber component differ greatlyfrom each other in polarity, and hence it is extremely difficult toselect an adhesive showing affinity for both.

[0011] In addition, even when an adhesive is selected which shows atleast some affinity for each of them, the adhesive layer is in directcontact with the photosensitive layer, and hence components in thephotosensitive resin layer migrate with time into the adhesive layerand, as a result, the properties of the adhesive are often inhibited.

[0012] When an initiator or a monomer in the photosensitive resin layermigrates into the adhesive layer, the problem exists that the monomercauses polymerization in the adhesive layer as well in the exposingstep, leaving curing contraction stress which may cause delamination atthe interface between the support and the adhesive layer.

[0013] Accordingly, the adhesion force between the support and thephotosensitive resin layer depends upon a combination of thecompositions of the two and, in order to obtain a strong adhesion force,sufficient consideration must be given to this.

[0014] However, it has been difficult to find a combination of aphotosensitive resin layer, an adhesive and a support, where thephotosensitive resin layer exhibits excellent mechanical properties andrelief-reproducing properties and yet retains strong adhesion to asupport.

[0015] An object of the present invention is to provide a photosensitiveconstituent for a flexographic printing plate which shows goodcapabilities of forming extremely fine halftone dots and providing goodprinting quality in process printing, which enables the tackiness of theprinting plate to be easily removed through post-exposure treatment,which avoids formation of cracks in the printing plate surface, in whichadhesion force between the photosensitive resin layer and its support ofa polyester film is strong enough not to cause any trouble under actualprinting conditions, and which is stable with time.

DISCLOSURE OF THE INVENTION

[0016] As a result of extensive investigations, the inventors have foundthat the above objects can be achieved by using the following novelphotosensitive elastomer composition in the photoconductive constituentfor a flexographic printing plate, thus resulting in the presentinvention.

[0017] That is, the present invention provides a photosensitiveconstituent for a flexographic printing plate comprising a polyesterfilm as support (B) and a photosensitive elastomer composition (C) andhaving an adhesive layer (A) therebetween,

[0018] wherein the photosensitive elastomer composition (C) comprises,as essential components, (a) 50% by weight or more of at least onethermoplastic elastomer comprising a mono-vinyl-substituted aromatichydrocarbon and a conjugated diene, (b) 5 to 40% by weight of at leastone diene-based liquid rubber, (c) 1 to 20% by weight of at least oneethylenically unsaturated compound having 2 unsaturated bonds within itsmolecule, and (d) 0.1 to 3% by weight of at least onephotopolymerization initiator, and

[0019] wherein (1) the thermoplastic elastomer has an average ratio ofvinyl bond units in conjugated diene segments thereof of 10 to 20 mol %and (2) the liquid rubber has a number average molecular weight (Mn) of500 to 5,000 and an average ratio of vinyl bond units of 50 to 70 mol %.

BEST MODE FOR CARRYING OUT THE INVENTION

[0020] The thermoplastic elastomer used in the present invention in acontent of 50% by weight or more, is obtained by polymerizing amono-vinyl-substituted aromatic hydrocarbon monomer and a conjugateddiene monomer. Styrene, α-methylstyrene, p-methylstyrene,p-methoxystyrene or the like is used as the mono-vinyl-substitutedaromatic hydrocarbon, and butadiene, isoprene or the like is used as theconjugated diene monomer. Typical examples of the elastomer includestyrene-butadiene-styrene block copolymer, styrene-isoprene-styreneblock copolymer, etc. As to the content of the mono-vinyl-substitutedaromatic hydrocarbon in the thermoplastic elastomer, a range of 8 to 40%by weight is preferred. If the content is too low, cold flow of thephotosensitive elastomer composition results, and a good thickness isnot obtained. If too high, the hardness of the resulting flexograpicprinting plate becomes too high, and good printing quality is notobtained.

[0021] Vinyl bond units in the conjugated diene segments of thethermoplastic elastomer contribute to improvement of reproducibility ofa relief but, at the same time, they can be the cause of increasingtackiness of the surface of the flexographic printing plate. From thestandpoint of balancing these two properties, the average ratio of vinylbond units is required to be 10 to 20%, preferably 12 to 20%, morepreferably 14 to 20%. The term “average ratio of vinyl bond units” asused herein means, in the conjugated diene segments using, for example,butadiene which segments include 1,2-bond segments and 1,4-bondsegments, an average ratio of side chain-vinyl group-containing segmentsgenerated from the 1,2-bond based on the mol number of the totalbutadiene used.

[0022] The average content of mono-vinyl-substituted aromatichydrocarbon and the conjugated diene can be determined by calculatingthe content of the mono-vinyl-substituted aromatic hydrocarbon. In thecase of, for example, styrene-butadiene type elastomers, it can bedetermined according to Hampton method using IR spectrum. In the case ofstyrene-isoprene elastomers, it can be determined from the followingformula, in which C represents an area of peak signal at 6.3 to 7.3 ppmin H¹-NMR spectrum originating from hydrogen on the benzene ring, and Drepresents the total area:Average  content  of  styrene  (mol%) = Sm = 100 × (8/5) × (C/D)$\begin{matrix}{{Average}\quad {content}\quad {of}\quad {styrene}} \\\left( {\% \quad {by}\quad {weight}} \right)\end{matrix} = {100 \times \frac{104 \times {Sm}}{{104 \times {Sm}} + {68 \times \left( {100 - {Sm}} \right)}}}$

[0023] Further, in the case of, for example, styrene-butadiene typeelastomers, the average ratio of vinyl bond units in the conjugateddiene segment of the thermoplastic elastomer is determined from IRspectrum of a 1% solution in carbon disulfide according to Hamptonmethod described in Anal. Chem., 21, 923 (1949). In the case ofstyrene-isoprene type elastomers, it is determined from the followingformula wherein A represents an area of peak signal at around 4.7 ppm inH¹-NMR spectrum originating from the vinyl bond, and B represents anarea of peak signal at around 5.1 ppm originating from 1,4-bond:

Average ratio of vinyl bond units(mol %)=100×A/(A+2B)

[0024] Additionally, in conducting these analyses on the photosensitiveelastomer composition, the thermoplastic elastomer alone must beseparated before the analyses.

[0025] The thermoplastic elastomer to be used in the present inventioncan be obtained by the process described in, for example, JP-A-63-27573(corresponding to U.S. Pat. No. 4,792,584)(The term “JP-A” as usedherein means an “unexamined published Japanese patent application”).That is, the content of the mono-vinyl-substituted aromatic hydrocarboncan be adjusted to be within the scope of the present invention bycontrolling the amount thereof.

[0026] In addition, the ratio of the vinyl bond units of the conjugateddiene can be adjusted by using a polar compound as a vinylating agent inthe polymerizing process in a hydrocarbon solvent using an organiclithium compound as an initiator, and controlling the reaction throughthe kind and amount of the polar compound, polymerization temperature,and the like.

[0027] As the hydrocarbon solvent to be used in the polymerization,there may be used aliphatic hydrocarbons such as butane, pentane,hexane, isopentane, heptane, octane and isooctane; alicyclichydrocarbons such as cyclopentane, methylcyclopentane, cyclohexane andmethylcyclohexane; and aromatic hydrocarbons such as benzene, tolueneand xylene. The organic lithium compound may be, for example, ethyllithium, propyl lithium, butyl lithium, hexamethylene dilithium, etc.

[0028] Further, exemplary polar compounds to be added for adjustingvinyl bond content, include ethers such as tetrahydrofuran, diethyleneglycol dimethyl ether, diethylene glycol dibutyl ether, etc.; aminessuch as triethylamine, tetramethylethylenediamine, etc.; thioethers;phosphines; alkylbenzenesulfonates; potassium or sodium alkoxides; andthe like.

[0029] As the diene-based liquid rubber in the present invention, liquidpolybutadiene, polyisoprene or the like having a number averagemolecular weight of 500 to 5,000 is used in an amount of 5 to 40% byweight based on the total amount of the photosensitive composition. Incases when the ratio of the liquid rubber is more than the above range,the resulting elastomer composition becomes soft, thus lackingdimensional stability at room temperature whereas, in cases when lessthan the above range, an insufficient plastic effect results, andflexibility of the resulting flexographic printing plate becomesinsufficient. Thus, the liquid rubber is required to be present in theabove-described range.

[0030] As to the average ratio of vinyl bond units in the liquid rubber,it must be in the range of from 50 to 70 mol %. If too low, it isdetrimental to the reproduction of a relief image whereas, if too high,the photocured resin is so fragile that the resulting relief is liableto be chipped. The ratio of vinyl bond units can be determined accordingto the Hampton method or the H¹-NMR method as is the same with thethermoplastic elastomer.

[0031] Further, exemplary ethylenically unsaturated compounds having 2unsaturated bonds include esters between a dihydric alcohol and acrylicacid or methacrylic acid such as butanediol di(meth)acrylate, hexanedioldi(meth)acrylate, nonanediol di(meth)acrylate, ethylene glycoldi(meth)acrylate, polyethylene glycol di(meth)acrylate, bisphenol Aethylene oxide adduct di(meth)acrylate and bisphenol A propylene oxideadduct di(meth)acrylate; reaction products between diisocyanate such ashexamethylenediisocyanate or trilenediisocyanate and a methacrylic oracrylic acid derivative having a hydroxyl group or an amino group in theside chain; and the like. Such compounds may be used alone or incombination, in a range of 1 to 20% by weight in the photosensitiveelastomer composition.

[0032] In particular, when the ethylenically unsaturated compoundincludes a compound represented by formula (1), there exist theadvantages that high exposure sensitivity can be obtained which enablesrelief exposure time to be shortened, and extremely fine halftonedot-forming properties result with corresponding print reproducibilitywith high quality being obtained.

[0033] The photoinitiator employed in the present invention may beselected from among known photopolymerization initiators includingaromatic ketones like benzophenone, benzoin methyl ether, benzoin ethylether, benzoin propyl ether, α-methylol benzoin methyl ether,α-methoxybenzoin methyl ether, 2,2-diethoxyphenylacetophenone,2,2-dimethoxy-1,2-diphenylethan-1-one,2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1, etc.Combinations of the above photoinitiators may be used. Thephotoinitiator may be used in an amount in a range of from 0.1 to 3.0%by weight.

[0034] Of these initiators, a combination of initiators of2,2-dimethoxy-1,2-diphenylethan-1-one and2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one or2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1 serves, whenused, to form a relief having a cross-sectional shape nearer toperpendicularity and reduces dot gain upon printing. In addition, thereare advantages that tackiness reduction after post-exposure treatment isimproved and that the surface suffers little formation of cracks, withdurability being improved.

[0035] The photosensitive elastomer composition of the present inventionmay further contain, as additional compounds, additives such as athermal polimerization inhibitor, a plasticizer, a colorant, etc.depending upon required properties.

[0036] As the composition of the adhesive layer, there may be used, forexample, a composition prepared by compounding a multi-functionalisocyanate with a mixture of a thermoplastic elastomer obtained bypolymerization between the mono-vinyl-substituted aromatic hydrocarbonmonomer and the conjugated diene monomer and polybutadiene part of whichis modified with hydroxyl group or carboxyl group; a composition of areaction product between a mixture comprising a high molecular compoundhaving an urethane or ester structure and a hydroxyl group-containingethylenically unsaturated compound and a multi-functional isocyanate;and the like.

[0037] A particularly preferred adhesive is a reaction product between amixture comprising polyol having a polyester structure, a polyurethanestructure or both of the structures and a hydroxyl group-containingethylenically unsaturated compound and a multi-functional isocyanate.

[0038] The polyol having a polyester structure is obtained bypolycondensation reaction between a diol component and a dicarboxylicacid component, with the diol component being employed in an excess. Asthe diol component, diols having a skeleton of a straight and branchedaliphatic saturated hydrocarbon, bisphenol A, a diol having a structureof an ethylene oxide adduct of bisphenol A, polyoxyalkylene glycols, andmixtures of these diols may be used. On the other hand, as thedicarboxylic acid component, dicarboxylic acids having a skeleton of astraight and branched aliphatic saturated hydrocarbon, terephthalicacid, isophthalic acid, and mixtures of these dicarboxylic acidcomponents may be used.

[0039] In addition, the polyol having a polyurethane structure can beobtained by polyaddition reaction between a diol component as describedabove and a diisocyanate compound, with the diol component being in anexcess amount.

[0040] The polyol having the structure of both polyester andpolyurethane can be obtained by charging an excess amount, based on thediisocyanate compound, of a previously synthesized polyol having apolyester structure to react.

[0041] Further, exemplary ethylenically unsaturated compounds containinga hydroxyl group, include, for example, 2-hydroxyethyl (meth)acrylate,2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate,2-hydroxy-3-acryloyloxypropyl (meth)acrylate), 2-hydroxy-3-phenoxypropyl(meth)acrylate, and the like.

[0042] The adhesive layer between the support and the photosensitiveresin layer can be obtained by dissolving a reaction product between amixture of the above-described polyol and the ethylenically unsaturatedcompound containing a hydroxyl group and the multi-functional isocyanatein a suitable solvent, and coating the solution, followed by drying.

[0043] Here, if the molecular weight of the polyol is too low, thereaction product thereof with the multi-functional isocyanate has such ahigh degree of tackiness that handling properties after coating on asupport suffer. On the other hand, from the point of view of reactivityof the multi-functional isocyanate, if the molecular weight of thepolyol is too high, there results a disadvantage that the urethanatingreaction becomes slow. Thus, the number average molecular weight of thepolyol is preferably 10,000 to 100,000 in terms of polystyrene accordingto gel permeation chromatography.

[0044] Exemplary multi-functional isocyanates include tolylenediisocyanate, 4,4-diphenylmethane diisocyanate, xylylene diisocyanate,hexamethylene diisocyanate, those compounds which are prepared byaddition reaction between these multi-functional isocyanates andmulti-functional alcohols such as pentaerythritol andtrimethylolpropane, with the multi-functional isocyanates being employedin excess.

[0045] Coating of the adhesive onto the support can be conducted, forexample, in the following manner. That is, an adhesive comprising amixture of 100 parts of the polyol component and 1 to 30 parts of theethylenically unsaturated compound containing a hydroxyl group and 1 to30 parts of the multi-functional isocyanate are dissolved in an organicsolvent such as ethyl acetate, methyl ethyl ketone, tetrahydrofuran ortoluene or a mixture thereof, uniformly coated on a support of polyesterfilm, then dried at 80 to 100° C. for several seconds (e.g. ten seconds)to several minutes to form an adhesive layer. In this occasion, a UVabsorbent, a dye, and the like may be added to the adhesive layer as thecase demands for the purpose of imparting antihalation effect. Theamount of the adhesive after drying is usually 5 to 30 g/m². Thereafter,in order to force the urethanation reaction to proceed in the adhesivelayer, it is left at 40 to 50° C. for several days.

[0046] In cases when a highly adhesive coating is additionally providedon the surface of the support of polyester film on which surface is tobe coated the adhesive, the adhesion force between the adhesive layerand the support becomes stronger, resulting in a more preferredconstituent for a flexographic printing plate.

[0047] The highly adhesive coating is usually laminated during the stepsof producing the polyester film, which is more advantageous incomparison with the case of providing an adhesive subbing layer in anoff-line manner in view of production cost and stability of quality.

[0048] As such polyester films having laminated on the highly adhesivecoating, various ones are commercially available for various coatingmaterials and inks, and a proper one effective in the present inventionas well may be selected from among them.

[0049] As examples of the commercially available polyester films havinglaminated thereon a highly adhesive coating, there are illustratedLumilar T90 series or Q80 series made by Toray Co., Ltd., CosmoshineA4000 series made by Toyobo K. K., Tetron HPE and SG2 made by TeijinCo., Ltd., Merinex 700 series made by E. I. du Pont de Nemours & Co.Inc., and the like.

[0050] The photosensitive constituent of the present invention for aflexographic printing plate may be prepared by various methods. Forexample, starting materials of the photosensitive elastomer compositionare dissolved in a suitable solvent such as chloroform,tetrachloroethylene, methyl ethyl ketone or toluene to be mixed, thencast into a frame, followed by evaporating the solvent to form a plate.Alternatively, the constituent may be formed by kneading in a kneader ora roll mill without using any solvent, and forming as a plate with adesired thickness on a support using an extruder, an injection moldingmachine, a press or the like.

[0051] Since the photosensitive elastomer composition usually has sometackiness, a thin film of polyethylene, polypropylene, polyester,polystyrene or the like is laminated on the surface of the resin layerin order to improve contact properties with a transparent image carrierto be superposed thereon upon plate making or permit re-use of anegative film. This film will be delaminated after completion ofexposure through a negative film superposed thereon.

[0052] For the same purpose, a solvent-soluble, thin, flexibleprotective layer (see, for example, JP-B-5-13305) may be provided inplace of the film. In addition, this flexible layer may be made into aUV radiation-blocking layer containing an infrared radiation-sensitivesubstance, and subjected to direct imagewise irradiation with aninfrared laser so that this flexible layer itself may be used as anegative. In both cases, this thin, flexible, protective layer isremoved simultaneously by washing away unexposed areas after completionof the exposure.

[0053] In the case of providing the solvent-soluble, thin, flexibleprotective layer such as a layer of polyamide, partially saponifiedpolyvinyl acetate, cellulose ester or the like soluble in a wash-outsolution on the surface of the photosensitive resin layer, this may besolved in a proper solvent, followed by directly coating the solutiononto the photosensitive resin layer. Alternatively, it may be coated ona film of polyester, polypropylene or the like (to form a protectivefilm), then this protective film transferred to the photosensitive layerby lamination or press bonding.

[0054] A photosensitive elastomer layer having the desired thickness canbe obtained by bringing, after formation of the sheet of thephotosensitive resin composition, the protective film or the supportinto close contact with the photosensitive elastomer composition by rolllamination, followed by heat-pressing.

[0055] Exemplary sources of UV radiation to be used for photocuring thephotosensitive elastomer layer, include a high pressure mercury lamp, aUV ray fluorescent lamp, a carbon arc lamp, a xenon lamp, sunlight, andthe like.

[0056] A desired relief image can be obtained by exposing thephotosensitive elastomer layer with UV radiation through a negative filmand, in order to obtain a relief image having greater stability againstpressure to be applied to the cured layer upon wash out of uncuredareas, it is effective to conduct all-over exposure from the supportside. Either the exposure from the negative film side or the exposurefrom the support side may be conducted first, or both exposures may beconducted at the same time but, from the standpoint of view of imagereproducibility, it is preferred to first conduct the exposure from thesupport side.

[0057] Exemplary developing solvents to be used for washing outunexposed areas after formation of an image by irradiating thephotosensitive resin layer with UV radiation through a transparent imagecarrier, include esters such as heptyl acetate and 3-methoxybutylacetate, petroleum fraction, hydrocarbons such as toluene and decalin,and a mixture of a chlorine-containing organic solvent such astetrachloroethylene and an alcohol such as propanol, butanol orpentanol. Wash out of unexposed areas is conducted by jetting through anozzle or brushing with a brush. The thus obtained printing plate isrinsed, dried, and subjected to irradiation of the surface with light of300 nm or less in wavelength, followed by, if necessary, post exposuretreatment with light of more than 300 nm, to finish.

[0058] The present invention will be illustrated in greater detail withreference to the following Examples.

EXAMPLES 1 TO 3 AND COMPARATIVE EXAMPLES 1 AND 2

[0059] (1) Preparation of Photosensitive Elastomer Composition:

[0060] As thermoplastic elastomer, those which were synthesizedaccording to the process described in JP-A-63-27573 and shown in Table 1were used. In addition, “%” represents “% by weight” unless otherwiseindicated. TABLE 1 Content of Type of thermoplastic elastomer Content ofStyrene Vinyl A Styrene-butadiene-styrene 40% 13 mol % BStyrene-butadiene-styrene 22% 30 mol % C Styrene-butadiene-styrene 24%12 mol % D Styrene-isoprene-styrene 14%  8 mol %

[0061] Photosensitive resin compositions were prepared by combining thethermoplastic elastomers of Table 1 so that the weight ratio of theelastomers was 60% as shown Table 2 to be shown hereinafter, employingas other components 29% of liquid polybutadiene (B-2000, made by NihonSekiyu Kagaku) having a number average molecular weight of 2000 and avinyl content of 65 mol %, 5% of dioctyl fumarate, 2% ofN-laurylmaleimide, 2% of alkane diol diacrylate (carbon number of diolcomponent: 14 to 16), 1.7% of 2,2-dimethoxy-1,2-diphenylethan-1-one and0.3% of 2,6-di-t-butyl-p-cresol, and kneading them in a kneader.

[0062] (2) Preparation of Support:

[0063] 624 g of neopentyl glycol, 93 g of ethylene glycol, 485 g ofsebacic acid and 382 g of isophthalic acid were reacted in an aerialatmosphere at a reaction temperature of 180 C under a reduced pressureof 10 mmHg for 6 hours, then 87 g of tolylene diisocyanate was addedthereto, followed by reacting at 80° C. for 5 hours. Number averagemolecular weight of the obtained polyol was measured by the gelpermeation chromatography to be about 32,000 in terms of polystyrene.

[0064] A mixture of 100 parts of this polyol and 2 parts of2-hydroxypropyl methacrylate and 17 parts of an adduct betweentrimethylolpropane (1 mol) and tolylenediisocyanate (3 mols) were addedto 300 parts of ethyl acetate to prepare a uniform solution, and coatedon a polyester film, A4100, having a thickness of 125 μm and made byToyobo K. K. in a dry amount of 10 to 14 g/m² using a knife coater. Thiswas dried at 80° C. for 2 minutes, then left in a 40° C. atmosphere for3 days to obtain a support having an adhesive layer.

[0065] (3) Preparation of Photosensitive Constituent for FlexographicPrinting Plate and Production of Printing Plate:

[0066] The photosensitive elastomer composition prepared in (1) wassandwiched between the support prepared in (2) and a 100-μm thickpolyester cover sheet having a 4- to 6-μm thick ethyl cellulose layer. Apressure of 200 kg/cm² was applied thereto at 130° C. for 4 minutes by apress using a 3-mm spacer to form a photosensitive constituent for aflexographic printing plate.

[0067] The cover sheet of the obtained phosotensitive consitituent for aflexographic printing plate was delaminated, a negative film was broughtinto close contact with the protective layer of ethyl cellulose providedon the photosensitive resin layer, all-over exposure of 300 mJ/cm² wasfirst conducted from the support side on an exposing machine AFP-1500(manufactured by Asahi Kasei Kogyo) using a UV radiation fluorescentlamp emitting light of 370 nm in central wavelength, then imagewiseexposure of 9,000 mJ/cm² was conducted through a transparent imagecarrier. Exposure intensity upon this was measured by means of a UVilluminometer model MO-2 made by Oak Seisakusho using a UV-35 filter:Intensity of UV radiation from the lamp on the lower side from which theback exposure was conducted was measured to be 4.0 mW/cm² on the glassplate, and intensity of UV radiation from the lamp on the upper sidefrom which the relief exposure was conducted was measured to be 7.8mW/cm². Subsequently, development was conducted at a liquid temperatureof 25° C. by means of a developing machine Quick Line 912 (made by AsahiKasei Kogyo) using Solvit (made by Polyfibron) as a developing solution,followed by drying at 60° C. for 1 hour.

[0068] Thereafter, the whole surface was exposed in an exposure amountof 1,200 mJ/cm using a sterilization lamp emitting light of 254 nm incentral wavelength, then post-exposed using a UV radiation fluorescentlamp in an exposure amount of 1,000 mJ/cm². Additionally, the amount ofpoxt exposure using the sterilization lamp was calculated from theilluminance measured using a UV-25 filter of the MO-2 machine.

[0069] The results of evaluating the thus obtained printing plates areshown in Table 2. TABLE 2 Printing Average Vinyl Bond Tacky FeelingReproducibility of Combination of Units in Diene Formability of on PlateDots, Fine Lines and Elastomers Segment(mol %) Halftone Dots SurfaceReverse Areas Example 1 A 45%, B 15% 18.1 1%-150 lines/inch no goodExample 2 B 20%, D 40% 14.5 1%-150 lines/inch no good Example 3 C 60%12.0 1%-150 lines/inch no good Comparative D 60% 8.0 3%-120 lines/inchno Reproduction of dots Example 1 were limited. Comparative A 30%, B 3024.3 1%-150 lines/inch yes good Example 2

[0070] As is shown in Table 2, as to the thus obtained flexographicprinting plates of Examples 1 to 3, extremely fine halftone dots of1%-150 lines were formed, shoulder of the relief was almostperpendicular, and printing reproducibility of halftone dots, finelines, reverse areas, and the like was good as well. In addition, theprinting plates did not give a tacky feeling and, even when extraneousmaterial adhered to the surface, it was easily removable. Further, whenprinting on the polyethylene film was conducted using an ink containingabout 15 vol % of an acetic acid ester, delamination between the supportand the cured resin layer due to shear stress between the plate cylinderand the impression cylinder did not take place and, even when a largeforce was applied upon dismounting the printing plate fixed with anadhesive double-coated tape from the plate cylinder with holding theportion of the cured resin layer after completion, the cured resin layerwas not broken and tightly adhered to the support. Subsequently, thisflexographic printing plate was stored under the condition of 40° C. forone month, again mounted on the printing machine, and printing wasconducted, with no delamination between the cured resin layer and thesupport being observed.

[0071] In addition, no cracks were observed on the surface before,during and after printing, and good printing quality was maintained inthe long-time printing.

[0072] On the other hand, in Comparative Example 1, reproducibility ofextremely fine halftone dots was poor, and there was attained areproduction of 3%-120 lines/inch at the best. In Comparative Example 2,tacky feeling remained after post exposure, and good printing productswere not obtained due to extraneous matters remaining on the platesurface.

[0073] In addition, when the amount of post exposure using thesterilization lamp was increased to 1500 mJ/cm² in order to remove thistacky feeling, a number of cracks were formed on the plate surface witha slight stress though the tacky feeling was decreased to some extent.

COMPARATIVE EXAMPLE 3

[0074] A photosensitive constituent for flexographic printing plate wasformed in the same manner as in Example 1 except for changing the liquidpolybutadiene B-2000 to Polyoil 110 (made by Nihon Zeon; Mn=1600 andvinyl content=1 mol % according to catalogue data) and a flexographicprinting plate was made in the same manner as in Example 1, but therewas attained a reproduction of only 3%-120 lines/inch.

EXAMPLES 4 AND 5

[0075] 70% of the thermoplastic elastomer used in Example 3, 20% of aliquid polybutadiene (B-2000; made by Nihon Sekiyu Kagaku) having anumber average molecular weight of 2,000 and a vinyl content of 65 mol%, 7% of a monomer shown in Table 3, 1.0% of2,2-dimethoxy-1,2-diphenylethan-1-one, 1.0% of2-methyl-[4-methylthiophenyl]]-2-morpholino-1-propanone and 1.0% of2,6-di-t-butyl-p-cresol were kneaded in a kneader, sandwiched betweenthe support used in Examples 1 to 3 and a 100-um thick, polyester-madecover sheet having a 4- to 6-μum polyamide layer. A pressure of 200kg/cm² was applied thereto at 130° C. for 4 minutes by a press using a3-mm spacer to form a photosensitive constituent for flexographicprinting plate.

[0076] When this was processed in the same procedures as in Examples 1to 3 to make a flexographic printing plate, the results shown in Table 3were obtained. In every case, formation of extremely fine halftone dotsof 1%-150 lines/inch was possible. The sample of Example 5 permitted alessened amount of relief exposure and a short-time exposure in order tocomplete the exposure and, in addition, it was possible to formextremely fine halftone dots of 1%-175 lines/inch by giving a largeramount of relief exposure. In both Examples 4 and 5, the tacky feelingwas removed by the post exposure of 1,200 to 2,000 mJ/cm² using thesterilization lamp, and no crackes were observed on the plate surface.Observation of the cross section of the relief image revealed that theshoulder was in an almost perpendicular shape, and printing quality wasextremely good. TABLE 3 Example 4 Example 5 polyethylene glycoldiacrylate nonane- (Light Ester 9EG, made by diol Monomer Kyouei Kagaku)di-acrylate Relief Exposure 9,000 mJ 6,000 mJ Amount Formability of1%-150 lines/inch 1%-150 Extremely lines/inch Fine Dots Tackiness ofPlate no no Surface Cracks Plate no no Surface Printing Quality goodgood

[0077] Industrial Applicability

[0078] There are obtained good fine halftone dot-forming properties andprinting quality in process printing. Tackiness of a plate surface caneasily be removed, and cracks are difficult to be formed. At the sametime, adhesion force between the photosensitive resin layer and thesupport of polyester film is enough strong not to cause troubles underactual printing conditions, and flexographic printing plates can beprepared which are stable over time.

1. A photosensitive constituent for a flexographic printing platecomprising a polyester film as support (B) and a photosensitiveelastomer composition (C) and having an adhesive layer (A) therebetween,wherein the photosensitive elastomer composition (C) comprises, asessential components, (a) 50% by weight or more of at least onethermoplastic elastomer comprising the reaction product of amono-vinyl-substituted aromatic hydrocarbon monomer and a conjugateddiene monomer, (b) 5 to 40% by weight of at least one diene-based liquidrubber, (c) 1 to 20% by weight of at least one ethylenically unsaturatedcompound having 2 unsaturated bonds within its molecule, and (d) 0.1 to3% by weight of at least one photopolymerization initiator, and wherein(1) the thermoplastic elastomer has an average ratio of vinyl bond unitsin conjugated diene segments thereof of 10 to 20 mol % and (2) theliquid rubber has a number average molecular weight (Mn) of 500 to 5,000and an average ratio of vinyl bond units of 50 to 70 mol %.
 2. Thephotosensitive constituent for a flexographic printing plate accordingto claim 1, wherein the average ratio of vinyl bond units in theconjugated diene segments of the thermoplastic elastomer is 14 to 20 mol%.
 3. The photosensitive constituent for a flexographic printing plateaccording to claim 1 or 2, which contains, as the ethylenicallyunsaturated compound (c) having 2 unsaturated bonds, a compoundrepresented by formula (1):


4. The photosensitive constituent for a flexographic printing plateaccording to claim 1 or 2, wherein said photopolymerization initiatorcomprises a combination of 2,2-dimethoxy-1,2-diphenylethan-1-one and2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one or2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1 is used asthe photopolymerization initiator (d).
 5. The photosensitive constituentfor a flexographic printing plate according to any one of claims 1 to 4,wherein the adhesive layer (A) is a reaction product between a mixtureof a polyol having polyester structure, polyurethane structure or bothstructures and an ethylenically unsaturated compound containing ahydroxyl group, and a multi-functional isocyanate.
 6. The photosensitiveconstituent for a flexographic printing plate according to claim 5,wherein the support (B) is a polyester film having laminated thereon ahighly adhesive coating.